M&S says

Don’t let the chill stop you looking your best, with our handy two pack of thermal vests making it easy to add extra warmth to any outfit without compromising on style. These vests have a build up shoulder design for extra warmth and have an elegant scoop neckline so you can wear them discreetly under shirts, t-shirts and dresses.

Care and Composition

35 days to return - You may return unused products within 35 days of purchase with a valid parcel summary document. Your items don't need to be in the original box or bag as long as they're securely packaged.How to return
1. FREE returns to select M&S stores
2. Easy returns from home. Simply print the returns label, attach it to the top of your parcel and insert the returns note in the parcel. We will automatically deduct the cost of return from your refund.
To print the returns label and return, simply click here.Tell me more about how to return items and the returns policy